Network Working Group B. Laurie
Internet-Draft G. Sisson
Intended status: Standards Track R. Arends
Expires: January 10, 2008 Nominet
D. Blacka
VeriSign, Inc.
July 9, 2007
DNSSEC Hashed Authenticated Denial of Existencedraft-ietf-dnsext-nsec3-11
Status of this Memo
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This Internet-Draft will expire on January 10, 2008.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
The Domain Name System Security Extensions (DNSSEC) introduced the
NSEC resource record (RR) for authenticated denial of existence.
This document introduces an alternative resource record, NSEC3, which
similarly provides authenticated denial of existence. However, it
also provides measures against zone enumeration and permits gradual
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Internet-Draft nsec3 July 20071. Introduction1.1. Rationale
The DNS Security Extensions included the NSEC RR to provide
authenticated denial of existence. Though the NSEC RR meets the
requirements for authenticated denial of existence, it introduces a
side-effect in that the contents of a zone can be enumerated. This
property introduces undesired policy issues.
The enumeration is caused by the set of NSEC records that exists in a
side signed zone. An NSEC record lists two names that are ordered
canonically, in order to show that nothing exists between the two
names. The complete set of NSEC records lists all the names in a
zone. It is trivial to enumerate the content of a zone by querying
for names that do not exist.
An enumerated zone can be used, for example, as a source of probable
e-mail addresses for spam, or as a key for multiple WHOIS queries to
reveal registrant data which many registries may have legal
obligations to protect. Many registries therefore prohibit copying
of their zone data; however, the use of NSEC RRs renders these
policies unenforceable.
A second problem is that the cost to cryptographically secure
delegations to unsigned zones is high, relative to the perceived
security benefit, in two cases: large, delegation-centric zones, and
zones where insecure delegations will be updated rapidly. In these
cases, the costs of maintaining the NSEC RR chain may be extremely
high and use of the "Opt-Out" convention may be more appropriate (for
these unsecured zones).
This document presents the NSEC3 Resource Record which can be used as
an alternative to NSEC to mitigate these issues.
Earlier work to address these issues include [I-D.jas-dnsext-no],
[I-D.ietf-dnsext-dnssec-opt-in] and [I-D.laurie-dnsext-nsec2v2].
1.2. Reserved Words
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
1.3. Terminology
The reader is assumed to be familiar with the basic DNS and DNSSEC
concepts described in [RFC1034], [RFC1035], [RFC4033], [RFC4034],
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[RFC4035] and subsequent RFCs that update them: [RFC2136], [RFC2181]
and [RFC2308].
The following terminology is used throughout this document:
Zone enumeration: the practice of discovering the full content of a
zone via successive queries. Zone enumeration was non-trivial
prior to the introduction of DNSSEC.
Original owner name: the owner name corresponding to a hashed owner
name.
Hashed owner name: the owner name created after applying the hash
function to an owner name.
Hash order: the order in which hashed owner names are arranged
according to their numerical value, treating the leftmost (lowest
numbered) octet as the most significant octet. Note that this
order is the same as the canonical DNS name order specified in
[RFC4034] when the hashed owner names are in base32 encoded with
Extended Hex Alphabet [RFC4648].
Empty non-terminal: a domain name that owns no resource records, but
has one or more subdomains that do.
Delegation: an NS RRSet with a name different from the current zone
apex (non-zone-apex), signifying a delegation to a child zone.
Secure delegation: a name containing a delegation (NS RRSet), and a
signed DS RRSet, signifying a delegation to a signed child zone.
Insecure delegation: a name containing a delegation (NS RRSet), but
lacking a DS RRSet, signifying a delegation to an unsigned child
zone.
Opt-Out NSEC3 resource record: an NSEC3 resource record which has
the Opt-Out flag set to 1.
Opt-Out zone: a zone with at least one Opt-Out NSEC3 RR.
Closest encloser: the longest existing ancestor of a name. See also
section 3.3.1 of [RFC4592].
Closest provable encloser: the longest ancestor of a name that can
be proven to exist. Note that this is only different from the
closest encloser in an Opt-Out zone.
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Next closer name: the name one label longer than the closest
provable encloser of a name.
Base32: the "Base 32 Encoding with Extended Hex Alphabet" as
specified in [RFC4648]. Note that trailing padding characters
("=") are not used in the NSEC3 specification.
To cover: An NSEC3 RR is said to "cover" a name if the hash of the
name or "next closer" name falls between the owner name and the
next hashed owner name of the NSEC3. In other words, if it proves
the nonexistence of the name, either directly or by proving the
nonexistence of an ancestor of the name.
To match: An NSEC3 RR is said to "match" a name if the owner name of
the NSEC3 RR is the same as the hashed owner name of that name.
2. Backwards Compatibility
This specification describes a protocol change that is not generally
backwards compatible with [RFC4033], [RFC4034] and [RFC4035]. In
particular, security-aware resolvers that are unaware of this
specification (NSEC3-unaware resolvers) may fail to validate the
responses introduced by this document.
In order to aid deployment, this specification uses a signaling
technique to prevent NSEC3-unaware resolvers from attempting to
validate responses from NSEC3-signed zones.
This specification allocates two new DNSKEY algorithm identifiers for
this purpose. Algorithm XX, DSA-NSEC3 [### RFC-editor update
required, temporarily, XX=131] is an alias for algorithm 3, DSA.
Algorithm YY, RSASHA1-NSEC3 [### RFC-editor update required,
temporarily, YY=133] is an alias for algorithm 5, RSASHA1. These are
not new algorithms, they are simply additional identifiers for the
existing algorithms.
Zones signed according to this specification MUST only use these
algorithm identifiers for their DNSKEY RRs. Because these new
identifiers will be unknown algorithms to existing, NSEC3-unaware
resolvers, those resolvers will then treat responses from the NSEC3
signed zone as insecure, as detailed in [RFC4035], section 5.2.
Security aware resolvers that are aware of this specification MUST
recognize the new algorithm identifiers and treat them as equivalent
to the algorithms that they alias.
A methodology for transitioning from a DNSSEC signed zone to a zone
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signed using NSEC3 is discussed in Section 10.4.
3. The NSEC3 Resource Record
The NSEC3 Resource Record (RR) provides authenticated denial of
existence for DNS Resource Record Sets.
The NSEC3 RR lists RR types present at the original owner name of the
NSEC3 RR. It includes the next hashed owner name in the hash order
of the zone. The complete set of NSEC3 RRs in a zone indicates which
RRSets exist for the original owner name of the RR and form a chain
of hashed owner names in the zone. This information is used to
provide authenticated denial of existence for DNS data. To provide
protection against zone enumeration, the owner names used in the
NSEC3 RR are cryptographic hashes of the original owner name
prepended as a single label to the name of the zone. The NSEC3 RR
indicates which hash function is used to construct the hash, which
salt is used, and how many iterations of the hash function are
performed over the original owner name. The hashing technique is
described fully in Section 5.
Hashed owner names of unsigned delegations may be excluded from the
chain. An NSEC3 RR whose span covers the hash of an owner name or
"next closer" name of an unsigned delegation is referred to as an
Opt-Out NSEC3 RR and is indicated by the presence of a flag.
The owner name for the NSEC3 RR is the base32 encoding of the hashed
owner name prepended as a single label to the name of the zone.
The type value for the NSEC3 RR is NN. [### RFC-editor update
required, the examples assume NN=50]
The NSEC3 RR RDATA format is class independent and is described
below.
The class MUST be the same as the class of the original owner name.
The NSEC3 RR SHOULD have the same TTL value as the SOA minimum TTL
field. This is in the spirit of negative caching [RFC2308].
3.1. RDATA Fields3.1.1. Hash Algorithm
The Hash Algorithm field identifies the cryptographic hash algorithm
used to construct the hash-value.
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The values for this field are defined in the NSEC3 hash algorithm
registry, described in Section 11.
3.1.2. Flags
The Flags field contains 8 one-bit flags that can be used to indicate
different processing. All undefined flags must be zero. The only
flag defined by this specification is the Opt-Out flag.
3.1.2.1. Opt-Out Flag
If the Opt-Out flag is set, the NSEC3 record covers zero or more
unsigned delegations.
If the Opt-Out flag is clear, the NSEC3 record covers zero unsigned
delegations.
The Opt-Out Flag indicates whether this NSEC3 RR may cover unsigned
delegations. It is the least significant bit in the Flags field.
See Section 6 for details about the use of this flag.
3.1.3. Iterations
The Iterations field defines the number of additional times the hash
function has been performed. More iterations result in greater
resiliency of the hash value against dictionary attacks, but at a
higher computational cost for both the server and resolver. See
Section 5 for details of the use of this field, and Section 10.3 for
limitations on the value.
3.1.4. Salt Length
The Salt Length field defines the length of the Salt field in octets,
ranging in value from 0 to 255.
3.1.5. Salt
The Salt field is appended to the original owner name before hashing
in order to defend against pre-calculated dictionary attacks. See
Section 5 for details on how the salt is used.
3.1.6. Hash Length
The Hash Length field defines the length of the Next Hashed Owner
Name field, ranging in value from 1 to 255 octets.
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The Next Hashed Owner Name field contains the next hashed owner name
in hash order. This value is in binary format. Given the ordered
set of all hashed owner names, the Next Hashed Owner Name field
contains the hash of an owner name that immediately follows the owner
name of the given NSEC3 RR. The value of the Next Hashed Owner Name
field in the last NSEC3 RR in the zone is the same as the hashed
owner name of the first NSEC3 RR in the zone in hash order. Note
that, unlike the owner name of the NSEC3 RR, the value of this field
does not contain the appended zone name.
3.1.8. Type Bit Maps
The Type Bit Maps field identifies the RRSet types which exist at the
original owner name of the NSEC3 RR.
3.2. NSEC3 RDATA Wire Format
The RDATA of the NSEC3 RR is as shown below:
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hash Alg. | Flags | Iterations |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Salt Length | Salt /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hash Length | Next Hashed Owner Name /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Type Bit Maps /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Hash Algorithm is a single octet.
Flags field is a single octet, the Opt-Out flag is the least
significant bit, as shown below:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| |O|
+-+-+-+-+-+-+-+-+
Iterations is represented as a 16-bit unsigned integer, with the most
significant bit first.
Salt Length is represented as an unsigned octet. Salt Length
represents the length of the Salt field in octets. If the value is
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zero, the following Salt field is omitted.
Salt, if present, is encoded as a sequence of binary octets. The
length of this field is determined by the preceding Salt Length
field.
Hash Length is represented as an unsigned octet. Hash Length
represents the length of the Next Hashed Owner Name field in octets.
The next hashed owner name is not base32 encoded, unlike the owner
name of the NSEC3 RR. It is the unmodified binary hash value. It
does not include the name of the containing zone. The length of this
field is determined by the preceding Hash Length field.
3.2.1. Type Bit Maps Encoding
The encoding of the Type Bit Maps field is the same as that used by
the NSEC RR, described in [RFC4034]. It is explained and clarified
here for clarity.
The RR type space is split into 256 window blocks, each representing
the low-order 8 bits of the 16-bit RR type space. Each block that
has at least one active RR type is encoded using a single octet
window number (from 0 to 255), a single octet bitmap length (from 1
to 32) indicating the number of octets used for the bitmap of the
window block, and up to 32 octets (256 bits) of bitmap.
Blocks are present in the NSEC3 RR RDATA in increasing numerical
order.
Type Bit Maps Field = ( Window Block # | Bitmap Length | Bitmap )+
where "|" denotes concatenation.
Each bitmap encodes the low-order 8 bits of RR types within the
window block, in network bit order. The first bit is bit 0. For
window block 0, bit 1 corresponds to RR type 1 (A), bit 2 corresponds
to RR type 2 (NS), and so forth. For window block 1, bit 1
corresponds to RR type 257, bit 2 to RR type 258. If a bit is set to
1, it indicates that an RRSet of that type is present for the
original owner name of the NSEC3 RR. If a bit is set to 0, it
indicates that no RRSet of that type is present for the original
owner name of the NSEC3 RR.
Since bit 0 in window block 0 refers to the non-existing RR type 0,
it MUST be set to 0. After verification, the validator MUST ignore
the value of bit 0 in window block 0.
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Bits representing Meta-TYPEs or QTYPEs as specified in [RFC2929]
(section 3.1) or within the range reserved for assignment only to
QTYPEs and Meta-TYPEs MUST be set to 0, since they do not appear in
zone data. If encountered, they must be ignored upon reading.
Blocks with no types present MUST NOT be included. Trailing zero
octets in the bitmap MUST be omitted. The length of the bitmap of
each block is determined by the type code with the largest numerical
value, within that block, among the set of RR types present at the
original owner name of the NSEC3 RR. Trailing octets not specified
MUST be interpreted as zero octets.
3.3. Presentation Format
The presentation format of the RDATA portion is as follows:
o The Hash Algorithm field is represented as an unsigned decimal
integer. The value has a maximum of 255.
o The Flags field is represented as an unsigned decimal integer.
The value has a maximum of 255.
o The Iterations field is represented as an unsigned decimal
integer. The value is between 0 and 65535, inclusive.
o The Salt Length field is not represented.
o The Salt field is represented as a sequence of case-insensitive
hexadecimal digits. Whitespace is not allowed within the
sequence. The Salt field is represented as "-" (without the
quotes) when the Salt Length field has value 0.
o The Hash Length field is not represented.
o The Next Hashed Owner Name field is represented as an unpadded
sequence of case-insensitive base32 digits, without whitespace.
o The Type Bit Maps field is represented as a sequence of RR type
mnemonics. When the mnemonic is not known, the TYPE
representation as described in [RFC3597] (section 5) MUST be used.
4. The NSEC3PARAM Record
The NSEC3PARAM RR contains the NSEC3 parameters (hash algorithm,
flags, iterations and salt) needed to calculate hashed owner names.
The presence of an NSEC3PARAM RR at a zone apex indicates that the
specified parameters may be used by authoritative servers to choose
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an appropriate set of NSEC3 RRs for negative responses.
If an NSEC3PARAM RR is present at the apex of a zone with a Flags
field value of zero, then there MUST be an NSEC3 RR using the same
hash algorithm, iterations and salt parameters present at every
hashed owner name in the zone. That is, the zone MUST contain a
complete set of NSEC3 RRs with the same hash algorithm, iterations
and salt parameters.
The owner name for the NSEC3PARAM RR is the name of the zone apex.
The type value for the NSEC3PARAM RR is MM. [### RFC-editor update
required, the examples assume MM=51]
The NSEC3PARAM RR RDATA format is class independent and is described
below.
The class MUST be the same as the NSEC3 RRs to which this RR refers.
4.1. RDATA Fields
The RDATA for this RR mirrors the first four fields in the NSEC3 RR.
4.1.1. Hash Algorithm
The Hash Algorithm field identifies the cryptographic hash algorithm
used to construct the hash-value.
The acceptable values are the same as the corresponding field in the
NSEC3 RR.
4.1.2. Flag Fields
The Opt-Out flag is not used and is set to zero.
All other flags are reserved for future use, and must be zero.
NSEC3PARAM RRs with a Flags field value other than zero MUST be
ignored.
4.1.3. Iterations
The Iterations field defines the number of additional times the hash
is performed.
Its acceptable values are the same as the corresponding field in the
NSEC3 RR.
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Internet-Draft nsec3 July 20074.1.4. Salt Length
The Salt Length field defines the length of the salt in octets,
ranging in value from 0 to 255.
4.1.5. Salt
The Salt field is appended to the original owner name before hashing.
4.2. NSEC3PARAM RDATA Wire Format
The RDATA of the NSEC3PARAM RR is as shown below:
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hash Alg. | Flags | Iterations |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Salt Length | Salt /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Hash Algorithm is a single octet.
Flags field is a single octet.
Iterations is represented as a 16-bit unsigned integer, with the most
significant bit first.
Salt Length is represented as an unsigned octet. Salt Length
represents the length of the following Salt field in octets. If the
value is zero, the Salt field is omitted.
Salt, if present, is encoded as a sequence of binary octets. The
length of this field is determined by the preceding Salt Length
field.
4.3. Presentation Format
The presentation format of the RDATA portion is as follows:
o The Hash Algorithm field is represented as an unsigned decimal
integer. The value has a maximum of 255.
o The Flags field is represented as an unsigned decimal integer.
The value has a maximum value of 255.
o The Iterations field is represented as an unsigned decimal
integer. The value is between 0 and 65535, inclusive.
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o The Salt Length field is not represented.
o The Salt field is represented as a sequence of case-insensitive
hexadecimal digits. Whitespace is not allowed within the
sequence. This field is represented as "-" (without the quotes)
when the Salt Length field is zero.
5. Calculation of the Hash
The hash calculation uses three of the NSEC3 RDATA fields: Hash
Algorithm, Salt, and Iterations.
Define H(x) to be the hash of x using the Hash Algorithm selected by
the NSEC3 RR, k to be the number of Iterations, and || to indicate
concatenation. Then define:
IH(salt, x, 0) = H(x || salt), and
IH(salt, x, k) = H(IH(salt, x, k-1) || salt), if k > 0
Then the calculated hash of an owner name is
IH(salt, owner name, iterations),
where the owner name is in the canonical form, defined as:
The wire format of the owner name where:
1. The owner name is fully expanded (no DNS name compression) and
fully qualified;
2. All uppercase US-ASCII letters are replaced by the corresponding
lowercase US-ASCII letters;
3. If the owner name is a wildcard name, the owner name is in its
original unexpanded form, including the "*" label (no wildcard
substitution);
This form is as defined in section 6.2 of [RFC4034].
The method to calculate the Hash is based on [RFC2898].
6. Opt-Out
In this specification, as in [RFC4033], [RFC4034] and [RFC4035], NS
RRSets at delegation points are not signed and may be accompanied by
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a DS RRSet. With the Opt-Out bit clear, the security status of the
child zone is determined by the presence or absence of this DS RRSet,
cryptographically proven by the signed NSEC3 RR at the hashed owner
name of the delegation. Setting the Opt-Out flag modifies this by
allowing insecure delegations to exist within the signed zone without
a corresponding NSEC3 RR at the hashed owner name of the delegation.
An Opt-Out NSEC3 RR is said to cover a delegation if the hash of the
owner name or "next closer" name of the delegation is between the
owner name of the NSEC3 RR and the next hashed owner name.
An Opt-Out NSEC3 RR does not assert the existence or non-existence of
the insecure delegations that it may cover. This allows for the
addition or removal of these delegations without recalculating or re-
signing RRs in the NSEC3 RR chain. However, Opt-Out NSEC3 RRs do
assert the (non)existence of other, authoritative RRSets.
An Opt-Out NSEC3 RR MAY have the same original owner name as an
insecure delegation. In this case, the delegation is proven insecure
by the lack of a DS bit in the type map and the signed NSEC3 RR does
assert the existence of the delegation.
Zones using Opt-Out MAY contain a mixture of Opt-Out NSEC3 RRs and
non-Opt-Out NSEC3 RRs. If an NSEC3 RR is not Opt-Out, there MUST NOT
be any hashed owner names of insecure delegations (nor any other RRs)
between it and the name indicated by the next hashed owner name in
the NSEC3 RDATA. If it is Opt-Out, it MUST only cover hashed owner
names or hashed "next closer" names of insecure delegations.
The effects of the Opt-Out flag on signing, serving, and validating
responses are covered in following sections.
7. Authoritative Server Considerations7.1. Zone Signing
Zones using NSEC3 must satisfy the following properties:
o Each owner name within the zone that owns authoritative RRSets
MUST have a corresponding NSEC3 RR. Owner names that correspond
to unsigned delegations MAY have a corresponding NSEC3 RR.
However, if there is not a corresponding NSEC3 RR, there MUST be
an Opt-Out NSEC3 RR that covers the "next closer" name to the
delegation. Other non-authoritative RRs are not represented by
NSEC3 RRs.
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o Each empty non-terminal MUST have a corresponding NSEC3 RR, unless
the empty non-terminal is only derived from an insecure delegation
covered by an Opt-Out NSEC3 RR.
o The TTL value for any NSEC3 RR SHOULD be the same as the minimum
TTL value field in the zone SOA RR.
o The Type Bit Maps field of every NSEC3 RR in a signed zone MUST
indicate the presence of all types present at the original owner
name, except for the types solely contributed by an NSEC3 RR
itself. Note that this means that the NSEC3 type itself will
never be present in the Type Bit Maps.
The following steps describe a method of proper construction of NSEC3
RRs. This is not the only such possible method.
1. Select the hash algorithm and the values for salt and iterations.
2. For each unique original owner name in the zone add an NSEC3 RR.
* If Opt-Out is being used, owner names of unsigned delegations
MAY be excluded.
* The owner name of the NSEC3 RR is the hash of the original
owner name, prepended as a single label to the zone name.
* The Next Hashed Owner Name field is left blank for the moment.
* If Opt-Out is being used, set the Opt-Out bit to one.
* For collision detection purposes, optionally keep track of the
original owner name with the NSEC3 RR.
* Additionally, for collision detection purposes, optionally
create an additional NSEC3 RR corresponding to the original
owner name with the asterisk label prepended (i.e., as if a
wildcard existed as a child of this owner name) and keep track
of this original owner name. Mark this NSEC3 RR as temporary.
3. For each RRSet at the original owner name, set the corresponding
bit in the Type Bit Maps field.
4. If the difference in number of labels between the apex and the
original owner name is greater than 1, additional NSEC3 RRs need
to be added for every empty non-terminal between the apex and the
original owner name. This process may generate NSEC3 RRs with
duplicate hashed owner names. Optionally, for collision
detection, track the original owner names of these NSEC3 RRs and
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create temporary NSEC3 RRs for wildcard collisions in a similar
fashion to step 1.
5. Sort the set of NSEC3 RRs into hash order.
6. Combine NSEC3 RRs with identical hashed owner names by replacing
them with a single NSEC3 RR with the Type Bit Maps field
consisting of the union of the types represented by the set of
NSEC3 RRs. If the original owner name was tracked, then
collisions may be detected when combining, as all of the matching
NSEC3 RRs should have the same original owner name. Discard any
possible temporary NSEC3 RRs.
7. In each NSEC3 RR, insert the next hashed owner name by using the
value of the next NSEC3 RR in hash order. The next hashed owner
name of the last NSEC3 RR in the zone contains the value of the
hashed owner name of the first NSEC3 RR in the hash order.
8. Finally, add an NSEC3PARAM RR with the same Hash Algorithm,
Iterations and Salt fields to the zone apex.
If a hash collision is detected, then a new salt has to be chosen and
the signing process restarted.
7.2. Zone Serving
This specification modifies DNSSEC-enabled DNS responses generated by
authoritative servers. In particular, it replaces the use of NSEC
RRs in such responses with NSEC3 RRs.
In the following response cases, the NSEC RRs dictated by DNSSEC
[RFC4035] are replaced with NSEC3 RRs that prove the same facts.
Responses that would not contain NSEC RRs are unchanged by this
specification.
When returning responses containing multiple NSEC3 RRs, all of the
NSEC3 RRs MUST use the same hash algorithm, iteration, and salt
values. The Flags field value MUST be either zero or one.
7.2.1. Closest Encloser Proof
For many NSEC3 responses a proof of the closest encloser is required.
This is a proof that some ancestor of the QNAME is the closest
encloser of QNAME.
This proof consists of (up to) two different NSEC3 RRs:
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o An NSEC3 RR that matches the closest (provable) encloser.
o An NSEC3 RR that covers the "next closer" name to the closest
encloser.
The first NSEC3 RR essentially proposes a possible closest encloser,
and proves that the particular encloser does, in fact, exist. The
second NSEC3 RR proves that the possible closest encloser is the
closest, and proves that QNAME (and any ancestors between QNAME and
the closest encloser) do not exist.
These NSEC3 RRs are collectively referred to as the "closest encloser
proof" in the subsequent descriptions.
For example, the closest encloser proof for the nonexistent
"alpha.beta.gamma.example." owner name might prove that
"gamma.example." is the closest encloser. This response would
contain the NSEC3 RR that matches "gamma.example.", and would also
contain the NSEC3 RR that covers "beta.gamma.example." (which is the
"next closer" name.)
It is possible, when using Opt-Out (Section 6), to not be able to
prove the actual closest encloser because it is, or is part of an
insecure delegation covered by an Opt-Out span. In this case,
instead of proving the actual closest encloser, the closest provable
encloser is used. That is, the closest enclosing authoritative name
is used instead. In this case, the set of NSEC3 RRs used for this
proof is referred to as the "closest provable encloser proof."
7.2.2. Name Error Responses
To prove the nonexistence of QNAME a closest encloser proof and an
NSEC3 RR covering the (nonexistent) wildcard RR at the closest
encloser MUST be included in the response. This collection of (up
to) three NSEC3 RRs proves both that QNAME does not exist and that a
wildcard that could have matched QNAME also does not exist.
For example, if "gamma.example." is the closest provable encloser to
QNAME, then a NSEC3 RR covering "*.gamma.example." is included in the
authority section of the response.
7.2.3. No Data Responses, QTYPE is not DS
The server MUST include the NSEC3 RR that matches QNAME. This NSEC3
RR MUST NOT have the bits corresponding to either the QTYPE or CNAME
set in its Type Bit Maps field.
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If there is an NSEC3 RR that matches QNAME, the server MUST return it
in the response. The bits corresponding with DS and CNAME MUST NOT
be set in the Type Bit Maps field of this NSEC3 RR.
If no NSEC3 RR matches QNAME, the server MUST return a closest
provable encloser proof for QNAME. The NSEC3 RR that covers the
"next closer" name MUST have the Opt-Out bit set (note that this is
true by definition - if the Opt-Out bit is not set, something has
gone wrong).
If a server is authoritative for both sides of a zone cut at QNAME,
the server MUST return the proof from the parent side of the zone
cut.
7.2.5. Wildcard No Data Responses
If there is a wildcard match for QNAME, but QTYPE is not present at
that name, the response MUST include a closest encloser proof for
QNAME and MUST include the NSEC3 RR that matches the wildcard. This
combination proves both that QNAME itself does not exist and that a
wildcard that matches QNAME does exist. Note that the closest
encloser to QNAME MUST be the immediate ancestor of the wildcard RR
(if this is not the case, then something has gone wrong).
7.2.6. Wildcard Answer Responses
If there is a wildcard match for QNAME and QTYPE, then, in addition
to the expanded wildcard RRSet returned in the answer section of the
response, proof that the wildcard match was valid must be returned.
This proof is accomplished by proving that both QNAME does not exist,
and that the closest encloser of the QNAME and the immediate ancestor
of the wildcard are the same (i.e., the correct wildcard matched).
To this end, the NSEC3 RR that covers the "next closer" name of the
immediate ancestor of the wildcard MUST be returned. It is not
necessary to return an NSEC3 RR that matches the closest encloser, as
the existence of this closest encloser is proven by the presence of
the expanded wildcard in the response.
7.2.7. Referrals to Unsigned Subzones
If there is an NSEC3 RR that matches the delegation name, then that
NSEC3 RR MUST be included in the response. The DS bit in the type
bit maps of the NSEC3 RR MUST NOT be set.
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If the zone is Opt-Out, then there may not be an NSEC3 RR
corresponding to the delegation. In this case, the closest provable
encloser proof MUST be included in the response. The included NSEC3
RR that covers the "next closer" name for the delegation MUST have
the Opt-Out flag set to one. (Note that this will be the case unless
something has gone wrong).
7.2.8. Responding to Queries for NSEC3 Owner Names
The owner names of NSEC3 RRs are not represented in the NSEC3 RR
chain like other owner names. As a result, each NSEC3 owner name is
covered by another NSEC3 RR, effectively negating the existence of
the NSEC3 RR. This is a paradox, since the existence of an NSEC3 RR
can be proven by its RRSIG RRSet.
If the following conditions are all true:
o The QNAME equals the owner name of an existing NSEC3 RR, and
o No RR types exist at the QNAME, nor at any descendant of QNAME.
Then the response MUST be constructed as a Name Error response
(Section 7.2.2). Or, in other words, the authoritative name server
will act, as if the owner name of the NSEC3 RR did not exist.
Note that NSEC3 RRs are returned as a result of an AXFR or IXFR
query.
7.2.9. Server Response to a Run-time Collision
If the hash of a non-existing QNAME collides with the owner name of
an existing NSEC3 RR, then the server will be unable to return a
response that proves that QNAME does not exist. In this case, the
server MUST return a response with an RCODE of 2 (server failure).
Note that with the hash algorithm specified in this document, SHA-1,
such collisions are highly unlikely.
7.3. Secondary Servers
Secondary servers (and perhaps other entities) need to reliably
determine which NSEC3 parameters (i.e., hash, salt and iterations)
are present at every hashed owner name, in order to be able to choose
an appropriate set of NSEC3 RRs for negative responses. This is
indicated by an NSEC3PARAM RR present at the zone apex.
If there are multiple NSEC3PARAM RRs present, there are multiple
valid NSEC3 chains present. The server must choose one of them, but
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may use any criteria to do so.
7.4. Zones Using Unknown Hash Algorithms
Zones that are signed according to this specification, but are using
an unrecognized NSEC3 hash algorithm value, cannot be effectively
served. Such zones SHOULD be rejected when loading. Servers SHOULD
respond with RCODE=2 (server failure) responses when handling queries
that would fall under such zones.
7.5. Dynamic Update
A zone signed using NSEC3 may accept dynamic updates [RFC2136].
However, NSEC3 introduces some special considerations for dynamic
updates.
Adding and removing names in a zone MUST account for the creation or
removal of empty non-terminals.
o When removing a name with a corresponding NSEC3 RR, any NSEC3 RRs
corresponding to empty non-terminals created by that name MUST be
removed. Note that more than one name may be asserting the
existence of a particular empty non-terminal.
o When adding a name that requires adding an NSEC3 RR, NSEC3 RRs
MUST also be added for any empty non-terminals that are created.
That is, if there is not an existing NSEC3 RR matching an empty
non-terminal, it must be created and added.
The presence of Opt-Out in a zone means that some additions or
delegations of names will not require changes to the NSEC3 RRs in a
zone.
o When removing a delegation RRSet, if that delegation does not have
a matching NSEC3 RR, then it was opted out. In this case, nothing
further needs to be done.
o When adding a delegation RRSet, if the "next closer" name of the
delegation is covered by an existing Opt-Out NSEC3 RR, then the
delegation MAY be added without modifying the NSEC3 RRs in the
zone.
The presence of Opt-Out in a zone means that when adding or removing
NSEC3 RRs, the value of the Opt-Out flag that should be set in new or
modified NSEC3 RRs is ambiguous. Servers SHOULD follow this set of
basic rules to resolve the ambiguity.
The central concept to these rules is that the state of the Opt-Out
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flag of the covering NSEC3 RR is preserved.
o When removing an NSEC3 RR, the value of the Opt-Out flag for the
previous NSEC3 RR (the one whose next hashed owner name is
modified) should not be changed.
o When adding an NSEC3 RR, the value of the Opt-Out flag is set to
the value of the Opt-Out flag of the NSEC3 RR that previously
covered the owner name of the NSEC3 RR. That is, the now previous
NSEC3 RR.
If the zone in question is consistent with its use of the Opt-Out
flag (that is, all NSEC3 RRs in the zone have the same value for the
flag) then these rules will retain that consistency. If the zone is
not consistent in the use of the flag (i.e., a partially Opt-Out
zone), then these rules will not retain the same pattern of use of
the Opt-Out flag.
For zones that partially use the Opt-Out flag, if there is a logical
pattern for that use, the pattern could be maintained by using a
local policy on the server.
8. Validator Considerations8.1. Responses with Unknown Hash Types
A validator MUST ignore NSEC3 RRs with unknown hash types. The
practical result of this is that responses containing only such NSEC3
RRs will generally be considered bogus.
8.2. Verifying NSEC3 RRs
A validator MUST ignore NSEC3 RRs with a Flag fields value other than
zero or one.
A validator MAY treat a response as bogus if the response contains
NSEC3 RRs that contain different values for hash algorithm,
iterations, or salt from each other.
8.3. Closest Encloser Proof
In order to verify a closest encloser proof, the validator MUST find
the longest name, X, such that
o X is an ancestor of QNAME that is matched by an NSEC3 RR present
in the response. This is a candidate for the closest encloser.
And:
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o The name one label longer than X (but still an ancestor of--or
equal to--QNAME) is covered by an NSEC3 RR present in the
response.
One possible algorithm for verifying this proof is as follows:
1. Set SNAME=QNAME. Clear the flag.
2. Check whether SNAME exists:
* If there is no NSEC3 RR in the response that matches SNAME
(i.e., an NSEC3 RR whose owner name is the same as the hash of
SNAME, prepended as a single label to the zone name), clear
the flag.
* If there is an NSEC3 RR in the response that covers SNAME, set
the flag.
* If there is a matching NSEC3 RR in the response and the flag
was set, then the proof is complete, and SNAME is the closest
encloser.
* If there is a matching NSEC3 RR in the response, but the flag
is not set, then the response is bogus.
3. Truncate SNAME by one label from the left, go to step 2.
Once the closest encloser has been discovered, the validator MUST
check that the NSEC3 RR that has the closest encloser as the original
owner name is from the proper zone. The DNAME type bit must not be
set and the NS type bit may only be set if the SOA type bit is set.
If this is not the case, it would be an indication that an attacker
is using them to falsely deny the existence of RRs for which the
server is not authoritative.
In the following descriptions, the phrase "a closest (provable)
encloser proof for X" means that the algorithm above (or an
equivalent algorithm) proves that X does not exist by proving that an
ancestor of X is its closest encloser.
8.4. Validating Name Error Responses
A validator MUST verify that there is a closest encloser proof for
QNAME present in the response and that there is an NSEC3 RR that
covers the wildcard at the closest encloser (i.e., the name formed by
prepending the asterisk label to the closest encloser.)
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The validator MUST verify that an NSEC3 RR that matches QNAME is
present and that both the QTYPE and the CNAME type are not set in its
Type Bit Maps field.
Note that this test also covers the case where the NSEC3 RR exists
because it corresponds to an empty non-terminal, in which case the
NSEC3 RR will have an empty Type Bit Maps field.
8.6. Validating No Data Responses, QTYPE is DS
If there is an NSEC3 RR that matches QNAME present in the response,
then that NSEC3 RR MUST NOT have the bits corresponding to DS and
CNAME set in its Type Bit Maps field.
If there is no such NSEC3 RR, then the validator MUST verify that a
closest provable encloser proof for QNAME is present in the response,
and that the NSEC3 RR that covers the "next closer" name has the Opt-
Out bit set.
8.7. Validating Wildcard No Data Responses
The validator MUST verify a closest encloser proof for QNAME and MUST
find an NSEC3 RR present in the response that matches the wildcard
name generated by prepending the asterisk label to the closest
encloser. Furthermore, the bits corresponding to both QTYPE and
CNAME MUST NOT be set in the wildcard matching NSEC3 RR.
8.8. Validating Wildcard Answer Responses
The verified wildcard answer RRSet in the response provides the
validator with a (candidate) closest encloser for QNAME. This
closest encloser is the immediate ancestor to the generating
wildcard.
Validators MUST verify that there is an NSEC3 RR that covers the
"next closer" name to QNAME present in the response. This proves
that QNAME itself did not exist and that the correct wildcard was
used to generate the response.
8.9. Validating Referrals to Unsigned Subzones
The delegation name in a referral is the owner name of the NS RRSet
present in the authority section of the referral response.
If there is an NSEC3 RR present in the response that matches the
delegation name, then the validator MUST ensure that the NS bit is
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set and that the DS bit is not set in the Type Bit Maps field of the
NSEC3 RR. The validator MUST also ensure that the NSEC3 RR is from
the correct (i.e., parent) zone. This is done by ensuring that the
SOA bit is not set in the Type Bit Maps field of this NSEC3 RR.
Note that the presence of an NS bit implies the absence of a DNAME
bit, so there is no need to check for the DNAME bit in the Type Bit
Maps field of the NSEC3 RR.
If there is no NSEC3 RR present that matches the delegation name,
then the validator MUST verify a closest provable encloser proof for
the delegation name. The validator MUST verify that the Opt-Out bit
is set in the NSEC3 RR that covers the "next closer" name to the
delegation name.
9. Resolver Considerations9.1. NSEC3 Resource Record Caching
Caching resolvers MUST be able to retrieve the appropriate NSEC3 RRs
when returning responses that contain them. In DNSSEC [RFC4035], in
many cases it is possible to find the correct NSEC RR to return in a
response by name (e.g., when returning a referral, the NSEC RR will
always have the same owner name as the delegation.) With this
specification, that will not be true, nor will a cache be able to
calculate the name(s) of the appropriate NSEC3 RR(s).
Implementations may need to use new methods for caching and
retrieving NSEC3 RRs.
9.2. Use of the AD Bit
The AD bit, as defined by [RFC4035], MUST NOT be set when returning a
response containing a closest (provable) encloser proof in which the
NSEC3 RR that covers the "next closer" name has the Opt-Out bit set.
This rule is based on what this closest encloser proof actually
proves: names that would be covered by the Opt-Out NSEC3 RR may or
may not exist as insecure delegations. As such, not all the data in
responses containing such closest encloser proofs will have been
cryptographically verified, so the AD bit cannot be set.
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Internet-Draft nsec3 July 200710.1. Domain Name Length Restrictions
Zones signed using this specification have additional domain name
length restrictions imposed upon them. In particular, zones with
names that, when converted into hashed owner names, exceed the 255
octet length limit imposed by [RFC1035] cannot use this
specification.
The actual maximum length of a domain name in a particular zone
depends on both the length of the zone name (versus the whole domain
name) and the particular hash function used.
As an example, SHA-1 produces a hash of 160 bits. The base-32
encoding of 160 bits results in 32 characters. The 32 characters are
prepended to the name of the zone as a single label, which includes a
length field of a single octet. The maximum length of the zone name,
when using SHA-1, is 222 octets (255 - 33).
10.2. DNAME at the Zone Apex
The DNAME specification [RFC2672] section 3 has a 'no-descendants'
limitation. If a DNAME RR is present at node N, there MUST be no
data at any descendant of N.
If N is the apex of the zone, there will be NSEC3 and RRSIG types
present at descendants of N. This specification updates the DNAME
specification to allow NSEC3 and RRSIG types at descendants of the
apex regardless of the existence of DNAME at the apex.
10.3. Iterations
Setting the number of iterations used allows the zone owner to choose
the cost of computing a hash, and so the cost of generating a
dictionary. Note that this is distinct from the effect of salt,
which prevents the use of a single precomputed dictionary for all
time.
Obviously the number of iterations also affects the zone owner's cost
of signing and serving the zone as well as the validator's cost of
verifying responses from the zone. We therefore impose an upper
limit on the number of iterations. We base this on the number of
iterations that approximates the cost of verifying an RRSet.
The limits, therefore, are based on the size of the smallest zone
signing key, rounded up to the nearest table value (or rounded down
if the key is larger than the largest table value.)
A zone owner MUST NOT use a value higher than shown in the table
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below for iterations for the given key size. A resolver MAY treat a
response with a higher value as insecure, after the validator has
verified that the signature over the NSEC3 RR is correct.
+----------+------------+
| Key Size | Iterations |
+----------+------------+
| 1024 | 150 |
| 2048 | 500 |
| 4096 | 2,500 |
+----------+------------+
This table is based on an approximation of the ratio between the cost
of an SHA-1 calculation and the cost of an RSA verification for keys
of size 1024 bits (150 to 1), 2048 bits (500 to 1) and 4096 bits
(2500 to 1).
The ratio between SHA-1 calculation and DSA verification is higher
(1500 to 1 for keys of size 1024). A higher iteration count degrades
performance, while DSA verification is already more expensive than
RSA for the same key size. Therefore the values in the table MUST be
used independent of the key algorithm.
10.4. Transitioning a Signed Zone from NSEC to NSEC3
When transitioning an already signed and trusted zone to this
specification, care must be taken to prevent client validation
failures during the process.
The basic procedure is as follows:
1. Transition all DNSKEYs to DNSKEYs using the algorithm aliases
described in Section 2. The actual method for safely and
securely changing the DNSKEY RRSet of the zone is outside the
scope of this specification. However, the end result MUST be
that all DS RRs in the parent use the specified algorithm
aliases.
After this transition is complete, all NSEC3-unaware clients will
treat the zone as insecure. At this point, the authoritative
server still returns negative and wildcard responses that contain
NSEC RRs.
2. Add signed NSEC3 RRs to the zone, either incrementally or all at
once. If adding incrementally, then the last RRSet added MUST be
the NSEC3PARAM RRSet.
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3. Upon the addition of the NSEC3PARAM RRSet, the server switches to
serving negative and wildcard responses with NSEC3 RRs according
to this specification.
4. Remove the NSEC RRs either incrementally or all at once.
10.5. Transitioning a Signed Zone From NSEC3 to NSEC
To safely transition back to a DNSSEC [RFC4035] signed zone, simply
reverse the procedure above:
1. Add NSEC RRs incrementally or all at once.
2. Remove the NSEC3PARAM RRSet. This will signal the server to use
the NSEC RRs for negative and wildcard responses.
3. Remove the NSEC3 RRs either incrementally or all at once.
4. Transition all of the DNSKEYs to DNSSEC algorithm identifiers.
After this transition is complete, all NSEC3-unaware clients will
treat the zone as secure.
11. IANA Considerations
This document updates the IANA registry "DOMAIN NAME SYSTEM
PARAMETERS" [http://www.iana.org/assignments/dns-parameters] in sub-
registry "TYPES", by defining two new types. Section 3 defines the
NSEC3 RR type NN, (value 50 suggested). Section 4 defines the
NSEC3PARAM RR type MM (value 51 suggested).
This document updates the IANA registry "DNS SECURITY ALGORITHM
NUMBERS - per [RFC4035]"
http://www.iana.org/assignments/dns-sec-alg-numbers]. Section 2
defines the aliases DSA-NSEC3 (XX) and RSASHA1-NSEC3 (YY) for
respectively existing registrations DSA and RSASHA1.
Since these algorithm numbers are aliases for existing algorithm
numbers, the flags that exist for the original algorithm are valid
for the alias algorithm.
This document creates a new IANA registry for NSEC3 flags. This
registry should be named "DNSSEC NSEC3 Flags". The initial contents
of this registry are:
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Internet-Draft nsec3 July 200712.1. Hashing Considerations12.1.1. Dictionary Attacks
The NSEC3 RRs are still susceptible to dictionary attacks (i.e. the
attacker retrieves all the NSEC3 RRs, then calculates the hashes of
all likely domain names, comparing against the hashes found in the
NSEC3 RRs, and thus enumerating the zone). These are substantially
more expensive than enumerating the original NSEC RRs would have
been, and in any case, such an attack could also be used directly
against the name server itself by performing queries for all likely
names, though this would obviously be more detectable. The expense
of this off-line attack can be chosen by setting the number of
iterations in the NSEC3 RR.
Zones are also susceptible to a pre-calculated dictionary attack --
that is, a list of hashes for all likely names is computed once, then
NSEC3 RR is scanned periodically and compared against the precomputed
hashes. This attack is prevented by changing the salt on a regular
basis.
The salt SHOULD be at least 64 bits long and unpredictable, so that
an attacker cannot anticipate the value of the salt and compute the
next set of dictionaries before the zone is published.
12.1.2. Collisions
Hash collisions between QNAME and the owner name of an NSEC3 RR may
occur. When they do, it will be impossible to prove the non-
existence of the colliding QNAME. However, with SHA-1, this is
highly unlikely (on the order of 1 in 2^160). Note that DNSSEC
already relies on the presumption that a cryptographic hash function
is second pre-image resistant, since these hash functions are used
for generating and validating signatures and DS RRs.
12.1.3. Using New or Unknown Hash Algorithms
Since validators are instructed to ignore NSEC3 RRs with unknown hash
algorithms, simply using a new or unknown hash algorithm directly
will lead to validation failures with clients that understand NSEC3
but do not understand the hash algorithm.
To prevent this, care must be taken to protect such clients. It is
suggested that a similar technique to the one being used in this
specification to protect older clients be employed (see Section 2.)
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Internet-Draft nsec3 July 200712.1.4. Using High Iteration Values
Since validators should treat responses containing NSEC3 RRs with
high iteration values as insecure, presence of just one signed NSEC3
RR with a high iteration value in a zone provides attackers with a
possible downgrade attack.
The attack is simply to remove any existing NSEC3 RRs from a
response, and replace or add a single (or multiple) NSEC3 RR that
uses a high iterations value to the response. Validators will then
be forced to treat the response as insecure. This attack would be
effective only when all of following conditions are met:
o There is at least one signed NSEC3 RR that uses a high iterations
value present in the zone.
o The attacker has access to one or more of these NSEC3 RRs. This
is trivially true when the NSEC3 RRs with high iterations values
are being returned in typical responses, but may also be true if
the attacker can access the zone via AXFR or IXFR queries, or any
other methodology.
Using a high number of iterations also introduces an additional
denial-of-service opportunity against servers, since servers must
calculate several hashes per negative or wildcard response.
12.2. Opt-Out Considerations
The Opt-Out Flag (O) allows for unsigned names, in the form of
delegations to unsigned zones, to exist within an otherwise signed
zone. All unsigned names are, by definition, insecure, and their
validity or existence cannot be cryptographically proven.
In general:
o Resource records with unsigned names (whether existing or not)
suffer from the same vulnerabilities as RRs in an unsigned zone.
These vulnerabilities are described in more detail in [RFC3833]
(note in particular sections 2.3, "Name Chaining" and 2.6,
"Authenticated Denial of Domain Names").
o Resource records with signed names have the same security whether
or not Opt-Out is used.
Note that with or without Opt-Out, an insecure delegation may be
undetectably altered by an attacker. Because of this, the primary
difference in security when using Opt-Out is the loss of the ability
to prove the existence or nonexistence of an insecure delegation
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within the span of an Opt-Out NSEC3 RR.
In particular, this means that a malicious entity may be able to
insert or delete RRs with unsigned names. These RRs are normally NS
RRs, but this also includes signed wildcard expansions (while the
wildcard RR itself is signed, its expanded name is an unsigned name).
Note that being able to add a delegation is functionally equivalent
to being able to add any RR type: an attacker merely has to forge a
delegation to name server under his/her control and place whatever
RRs needed at the subzone apex.
While in particular cases, this issue may not present a significant
security problem, in general it should not be lightly dismissed.
Therefore, it is strongly RECOMMENDED that Opt-Out be used sparingly.
In particular, zone signing tools SHOULD NOT default to using Opt-
Out, and MAY choose to not support Opt-Out at all.
12.3. Other Considerations
Walking the NSEC3 RRs will reveal the total number of RRs in the zone
(plus empty non-terminals), and also what types there are. This
could be mitigated by adding dummy entries, but certainly an upper
limit can always be found.
13. References13.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, April 1997.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, March 1998.
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;; NSEC3 RR that matches the closest encloser (x.w.example)
;; H(x.w.example) = b4um86eghhds6nea196smvmlo4ors995
b4um86eghhds6nea196smvmlo4ors995.example. NSEC3 1 1 12 aabbccdd (
gjeqe526plbf1g8mklp59enfd789njgi MX RRSIG )
b4um86eghhds6nea196smvmlo4ors995.example. RRSIG NSEC3 133 2 3600 (
20150420235959 20051021000000 62827 example.
GWDmUk8Sv0dxy/UZFol4Ss7Wz3wBiongcnVy
strNODWwdnoO9z6pDh8JLk58ExfEgXm79i4b
Ma6C/s/bkk1LvA== )
;; NSEC3 RR that covers wildcard at the closest encloser (*.x.w.example)
;; H(*.x.w.example) = 92pqneegtaue7pjatc3l3qnk738c6v5m
35mthgpgcu1qg68fab165klnsnk3dpvl.example. NSEC3 1 1 12 aabbccdd (
b4um86eghhds6nea196smvmlo4ors995 NS DS RRSIG )
35mthgpgcu1qg68fab165klnsnk3dpvl.example. RRSIG NSEC3 133 2 3600 (
20150420235959 20051021000000 62827 example.
QrjOpXVIvodCw0O8uPMNA+yEeS/o3KKkEIPX
r5DoEShq2hymAsRTc/t9BvRKpcSTExyc5m3T
vYN3GgN0W/0WHQ== )
;; Additional
;; (empty)
The query returned three NSEC3 RRs that prove that the requested data
does not exist and that no wildcard expansion applies. The negative
response is authenticated by verifying the NSEC3 RRs. The
corresponding RRSIGs indicate that the NSEC3 RRs are signed by an
"example" DNSKEY of algorithm 133 and with key tag 62827. The
resolver needs the corresponding DNSKEY RR in order to authenticate
this answer.
One of the owner names of the NSEC3 RRs matches the closest encloser.
One of the NSEC3 RRs prove that there exists no longer name. One of
the NSEC3 RRs prove that there exists no wildcard RRSets that should
have been expanded. The closest encloser can be found by applying
the algorithm in section Section 8.3.
In the above example, the name 'x.w.example' hashes to
'b4um86eghhds6nea196smvmlo4ors995'. This indicates that this might
be the closest encloser. To prove that 'c.x.w.example' and
'*.x.w.example' do not exist, these names are hashed to,
respectively, '0va5bpr2ou0vk0lbqeeljri88laipsfh' and
'92pqneegtaue7pjatc3l3qnk738c6v5m'. The first and last NSEC3 RRs
prove that these hashed owner names do not exist.
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Internet-Draft nsec3 July 2007B.4. Wildcard Expansion
A query that was answered with a response containing a wildcard
expansion. The label count in the RRSIG RRSet in the answer section
indicates that a wildcard RRSet was expanded to produce this
response, and the NSEC3 RR proves that no "next closer" name exists
in the zone.
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The query returned an answer that was produced as a result of
wildcard expansion. The answer section contains a wildcard RRSet
expanded as it would be in a traditional DNS response. The RRSIG
Labels field value of 2 indicates that the answer is the result of
wildcard expansion, as the "a.z.w.example" name contains 4 labels.
This also shows that "w.example" exists, so there is no need for an
NSEC3 RR that matches the closest encloser.
The NSEC3 RR proves that no closer match could have been used to
answer this query.
B.5. Wildcard No Data Error
A "no data" response for a name covered by a wildcard. The NSEC3 RRs
prove that the matching wildcard name does not have any RRs of the
requested type and that no closer match exists in the zone.
;; Header: QR AA DO RCODE=0
;;
;; Question
a.z.w.example. IN AAAA
;; Answer
;; (empty)
;; Authority
example. SOA ns1.example. bugs.x.w.example. 1 3600 300 (
3600000 3600 )
example. RRSIG SOA 133 1 3600 20150420235959 20051021000000 (
62827 example.
hNIkW1xzn+c+9P3W7PUVVptI72xEmOtn+eqQ
ux0BE7Pfc6ikx4m7ivOVWETjbwHjqfY0X5G+
rynLZNqsbLm40Q== )
;; NSEC3 RR that matches the closest encloser (w.example)
;; H(w.example) = k8udemvp1j2f7eg6jebps17vp3n8i58h
k8udemvp1j2f7eg6jebps17vp3n8i58h.example. NSEC3 1 1 12 aabbccdd (
kohar7mbb8dc2ce8a9qvl8hon4k53uhi )
k8udemvp1j2f7eg6jebps17vp3n8i58h.example. RRSIG NSEC3 133 2 3600 (
20150420235959 20051021000000 62827 example.
IKJfInxfypsDiXKgT6HDvCPEIBu9lZCc0CWl
c46+Gj/Jrg1NBkSJkKMjCERp1HT8tKU+zYp5
Kyio/cddEaa5Gg== )
;; NSEC3 RR that covers the "next closer" name (z.w.example)
;; H(z.w.example) = qlu7gtfaeh0ek0c05ksfhdpbcgglbe03
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of salt, the cost of a precomputed dictionary doubles (because there
must be an entry for each word combined with each possible salt
value). The NSEC3 RR can use a maximum of 2040 bits (255 octets) of
salt, multiplying the cost by 2^2040. This means that an attacker
must, in practice, recompute the dictionary each time the salt is
changed.
Including a salt, regardless of size, does not affect the cost of
constructing NSEC3 RRs. It does increase the size of the NSEC3 RR.
There MUST be at least one complete set of NSEC3 RRs for the zone
using the same salt value.
The salt SHOULD be changed periodically to prevent pre-computation
using a single salt. It is RECOMMENDED that the salt be changed for
every re-signing.
Note that this could cause a resolver to see RRs with different salt
values for the same zone. This is harmless, since each RR stands
alone (that is, it denies the set of owner names whose hashes, using
the salt in the NSEC3 RR, fall between the two hashes in the NSEC3
RR) - it is only the server that needs a complete set of NSEC3 RRs
with the same salt in order to be able to answer every possible
query.
There is no prohibition with having NSEC3 RRs with different salts
within the same zone. However, in order for authoritative servers to
be able to consistently find covering NSEC3 RRs, the authoritative
server MUST choose a single set of parameters (algorithm, salt, and
iterations) to use when selecting NSEC3 RRs.
C.2. Hash Collision
Hash collisions occur when different messages have the same hash
value. The expected number of domain names needed to give a 1 in 2
chance of a single collision is about 2^(n/2) for a hash of length n
bits (i.e. 2^80 for SHA-1). Though this probability is extremely
low, the following paragraphs deal with avoiding collisions and
assessing possible damage in the event of an attack using hash
collisions.
C.2.1. Avoiding Hash Collisions During Generation
During generation of NSEC3 RRs, hash values are supposedly unique.
In the (academic) case of a collision occurring, an alternative salt
MUST be chosen and all hash values MUST be regenerated.
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Internet-Draft nsec3 July 2007C.2.2. Second Preimage Requirement Analysis
A cryptographic hash function has a second-preimage resistance
property. The second-preimage resistance property means that it is
computationally infeasible to find another message with the same hash
value as a given message, i.e. given preimage X, to find a second
preimage X' != X such that hash(X) = hash(X'). The work factor for
finding a second preimage is of the order of 2^160 for SHA-1. To
mount an attack using an existing NSEC3 RR, an adversary needs to
find a second preimage.
Assuming an adversary is capable of mounting such an extreme attack,
the actual damage is that a response message can be generated which
claims that a certain QNAME (i.e. the second pre-image) does exist,
while in reality QNAME does not exist (a false positive), which will
either cause a security aware resolver to re-query for the non-
existent name, or to fail the initial query. Note that the adversary
can't mount this attack on an existing name but only on a name that
the adversary can't choose and does not yet exist.
Authors' Addresses
Ben Laurie
Nominet
17 Perryn Road
London W3 7LR
England
Phone: +44 20 8735 0686
Email: ben@links.org
Geoffrey Sisson
Nominet
Sandford Gate
Sandy Lane West
Oxford OX4 6LB
UNITED KINGDOM
Phone: +44 1865 332211
Email: geoff@nominet.org.uk
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